This invention relates in general to web tracking apparatus including a web-supporting steering roller, and more particularly to web tracking apparatus in which a web-supporting steering roller is responsive to lateral movement of the supported web relative to the steering roller for angular adjustment of such roller.
In high speed electrographic reproduction apparatus for example, it is common practice to employ an elongated dielectric belt or web traveling along a path in operative relation with electrographic process stations. Such stations produce transferable images on the web and transfer such images from the web to receiver sheets to produce desired reproductions. Typically the web is supported by, and driven about, at least one roller. If the web were of a perfect construction and the support roller were perfectly cylindrical and mounted for rotation about its longitudinal axis, the velocity vector of the web would be substantially normal to the longitudinal axis of the roller. However, in practice, the velocity vector of the traveling web frequently approaches the longitudinal axis of the roller at an angle which produces lateral movement of the web relative to the roller.
Various apparatus for correcting for the lateral (cross-track) movement of the web have heretofore been utilized. Such apparatus include for example crowned rollers, flanged rollers, or servo actuated steering rollers. However, crown or flanged rollers are not suitable for use with a web in an electrographic reproduction apparatus. Crown rollers force the web toward the apex of such rollers and cause distortion of the web thereby producing local stresses in the web at the crown. Such stresses can damage the web. Meanwhile flanged rollers produce concentrated loading at the edges of the web which result in edge buckling, seam splitting, or excessive edge wear. On the other hand, while servo actuated steering rollers are suitable for use in electrographic reproduction apparatus, they are generally complex and costly.
Recently self-activated steering rollers have been developed for use in electrographic reproduction apparatus. Such steering rollers are simpler in construction and less costly than the servo actuated steering rollers. Examples of web tracking apparatus employing self-activated steering rollers are shown in U.S. Pat. Nos. 3,435,693; 3,540,571; and 4,397,538. The apparatus of such patents respectively sense lateral movement of a web and convert such movement into corresponding adjustment of the steering roller. However, with the web supported on the steering roller, adjustment of the steering roller moves the web laterally and such movement is immediately reflected in further movement of the sensing mechanism and readjustment of the steering roller. This may result in significant over corrections in steering roller adjustment with undue back-and-forth lateral movement of the web as it is realigned.